Published British Application No. 2,044,321A discloses a copolymer additive prepared from (1) a meth(acrylamido alkyl sulfonic acid) or alkali metal salt thereof and (2) a (meth)acrylamide or N-alkyl (meth)-acrylamide. The copolymer may be cross-linked with a quaternary ammonium salt.
U.S. Pat. Nos. 3,639,208 and 4,171,417 disclose poly-amphoteric polymers.